The invention relates to pen shaped syringes comprising a proximal part containing a dose setting and driving mechanism, a piston rod having a non-circular cross section and carrying along one side a cogging engaged by a carrier unidirectonally transmitting axial movements from the driving mechanism to the piston rod, a piston rod guide positioned to maintain the piston rod in a rotational position in the syringe ensuring the engagement between the carrier of the drive mechanism and the cogging of the piston rod, the piston rod guide having a non-circular opening conforming to the cross section of the piston rod, and through which opening the piston rod passes, and a distal part containing a tubular reservoir for medicine, which reservoir at its distal end is closed by a pierceable rubber membrane, and at its proximal end is closed by a piston actuated by the piston rod of the proximal part.